Calculation Dimension Last 30 Days

I'm using Marco Russo's DateTool Dimension example and running into issues trying to create a "Last 30 Days" calculation. Below is what I'm using. It works at the "Date" level in the hierarchy, but any higher up and it's aggregating incorrectly. Any thoughts?

( [Date Calculation].[Aggregation].[Last 30 Days] ) 
          = Aggregate(
                     { [Date Calculation].[Aggregation].DefaultMember } *
                     { ParallelPeriod(
                                       [Accounting Date].[Year-Quarter-Month-Date].[Month],
                                       1,
                                       [Accounting Date].[Year-Quarter-Month-Date]
                       ) : [Accounting Date].[Year-Quarter-Month-Date].CurrentMember
                     } );